I. Field of the Invention
The present invention generally relates to methods for improving the growth and crop productivity of plants by adjusting plant hormone levels and/or ratios. These methods are also useful for improving the resistance of plants to infestation by insects and pathogens, while, at the same time, improving plant growth by controlling plant hormones. More specifically, the present invention is directed to methods for achieving those goals by applying an effective amount of one or more plant hormones to the plant tissue. Alternatively, these goals are achieved by applying to the plant tissue other substances that effect the level of one or more plant hormones in the plant tissue, causing the hormone(s) to move into a desired range.
II. Description of the Background
Plant hormones have been known and studied for years. Plant hormones may be assigned to one of five categories: auxins, cytokinins, gibberellins, abscisic acid and ethylene. Ethylene has long been associated with fruit ripening and leaf abscission. Abscisic acid causes the formation of winter buds, triggers seed dormancy, controls the opening and closing of stomata and induces leaf senescence. Gibberellins, primarily gibberellic acid, are involved in breaking dormancy in seeds and in the stimulation of cell elongation in stems. Gibberellins are also known to cause dwarf plants to elongate to normal size. Cytokinins, e.g., zeatin, are produced primarily in the roots of plants. Cytokinins stimulate growth of lateral buds lower on the stem, promote cell division and leaf expansion and retard plant aging. Cytokinins also enhance auxin levels by creating new growth from meristematic tissues in which auxins are synthesized. Auxins, primarily indole-3-acetic acid (IAA) promote both cell division and cell elongation, and maintain apical dominance. Auxins also stimulate secondary growth in the vascular cambium, induce the formation of adventitious roots and promote fruit growth.
Auxins and cytokinins have complex interactions. It is known that the ratio of auxin to cytokinin will control the differentiation of cells in tissue cultures. Auxin is synthesized in the shoot apex, while cytokinin is synthesized mostly in the root apex. Thus, the ratio of auxin to cytokinin is normally high in the shoots, while it is low in the roots. If the ratio of auxin to cytokinin is altered by increasing the relative amount of auxin, root growth is stimulated. On the other hand, if the ratio of auxin to cytokinin is altered by increasing the relative amount of cytokinin, shoot growth is stimulated.
The most common naturally occurring auxin is indole-3-acetic acid (IAA). However, other synthetic auxins, including indole-3-butyric acid (IBA); naphthalene acetic acid (NAA); 2,4-dichlorophenoxy acetic acid (2,4-D); and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T or agent orange) are known. While these are recognized as synthetic auxins, it should be acknowledged that IBA does naturally occur in plant tissues. Many of these synthetic auxins have been employed for decades as herbicides, producing accelerated and exaggerated plant growth followed by plant death. Agent orange gained widespread recognition when it was used extensively by the United States Army and Air Force in deforestation applications during the Vietnam War. 2,4-D finds continuing use in a number of commercial herbicides sold for use by the home gardener.
Compounds are classified as auxins based on their biological activity in plants. A primary activity for classification includes simulation of cell growth and elongation. Auxins have been studied since the 1800's. Charles Darwin noticed that grass coleoptiles would grow toward a uni-directional light source. He discovered that the growth response of bending toward the light source occurred in the growth zone below the plant tip, even though it was the tip that perceived the light stimulus. Darwin suggested that a chemical messenger was transported between the plant tip and the growth zone. That messenger was later identified as an auxin.
All plants require a certain ratio of auxin, i.e., IAA, to cytokinin for cell division. While the ratios may vary, it is well known that the ratio of IAA to cytokinin must be much greater for cell division in the apical meristem tissue than the ratio in the meristem tissue of the roots. Each part of a plant may require a different IAA to cytokinin ratio for cell division. For example, different ratios may be required for cell division in the stem, fruit, grain and other plant parts. In fact, it has been estimated that the ratio for apical meristem cell division may be considerably more, in fact, as much as 1000 times greater than the ratio necessary for root cell division. While the mechanism by which this ratio is determined remains unknown, other hormones and enzymes are likely to be involved in its perception.
Plants generally grow best at temperatures from about 68° F. to about 87° F. (about 21-30° C.). In this temperature range it is presumed that plants produce sufficient amounts of auxins, particularly IAA, to maintain normal growth. While ideal temperatures vary among species, crop plants typically grow best in the foregoing range. While temperature is an important factor, it should also be noted that other environmental factors can effect cell division. The moisture content of the plant, the nutrient status (especially the level of available nitrogen), the light intensity on the plant and the age of the plant, together with the temperature, all effect the ability of the plant to produce plant hormones, including IAA and cytokinin which dictate cell division.
As the temperature rises above about 90° F. (above about 31° C.) or falls below about 68° F. (21° C.) plant growth and cell division slow. As the temperature further increases above about 90° F. and drops below about 68° F., the production of IAA and other plant hormones decreases at an accelerating rate. Thus, it becomes difficult, if not impossible, to achieve new cell growth at temperatures above about 100° F. Similarly cell division slows and then ceases as temperatures plunge significantly below about 68° F. During normal growing conditions with adequate moisture and temperature, i.e., temperatures between about 70° F. and 90° F., the plants will produce an abundance of IAA. Cell division may be further impeded by other inhibitive compounds produced by IAA and other plant hormones. As temperatures increase above about 90° F. or below about 68° F., the ability of plants to produce IAA rapidly diminishes.
Plants respond to light during the growth process. The light in the range of the red wavelengths is primarily used by plants in order to trigger normal plant growth. It also determines the plant's photoperiodism. When plants are spaced at relatively high density in a field, red wavelength light is reduced on plant parts by the shading effect of neighboring plants. This causes the shaded plant to seek out more sunlight and causes the extension of internode length as the shaded plant rapidly grows to seek more sunlight. It is well known that auxin (particularly IAA) moves from the light side of plant tissue to the dark side. When shading of lower plant parts becomes prominent in a field of plants, the movement of IAA from the new apical meristem tissue rapidly accelerates downward in the plant. The movement of IAA downward will be dependent upon the amount of shade that occurs at the bottom of the plant.
Since gibberellic acid tends to migrate in a plant to where there is the most abundance of red wavelength light, it will tend to move upward in a plant toward the apical meristem tissue. This, in turn, triggers the more rapid movement downward of IAA toward the shaded side of the plant. The amount of movement of IAA downward will depend upon the positioning of the apical meristem tissue of the plant. If the apical meristem tissue is located more vertically from the plant crown, IAA movement downward will be greater. If the apical meristem tissue is located more horizontally relative to the plant crown, IAA movement will be less. If the apical meristem tissue on a branch or a limb is bent downward, it is very difficult for IAA to move against gravity and therefore its movement downward will be limited.
When plants are rapidly growing under conditions that include ample moisture, ideal temperatures and ample amounts of nitrogen fertilizer, auxins are efficiently transported out of the tissues where they are metabolized and move downward in the plant. This results in the redistribution of auxin and the reduction of the auxin level in the tissues where it was produced. The result is tissues that are deficient in the level of auxin.
The present invention is based upon the Stoller model for plant growth. This model was developed from a combination of field observations and analysis of the scientific literature. This model takes into account published data on plant hormone levels and relates them to plant growth that can be observed to result from changes in these hormone levels. Although much research has been done over the past century on plant hormones, this is to our knowledge the first comprehensive model relating levels of hormones directly to field-observed plant growth responses. This model also provides for the first time an applicable method for controlling plant growth in the field with natural plant hormones to generate desired growth. Although there is a broad research base in the literature, most of this research deals with only one hormone or the specifics of the interaction of a subset of hormones within a very defined event. In addition most of this published work has been done in the laboratory on model plants, or has been done in vitro in excised or disrupted plant tissues. Never before has a model been published that relates the wide array of hormone responses to one another within developmental events with an eye to altering these responses to affect crop production by generating more ideal growth.
Ideal plant growth is defined as growth that would occur under conditions of ideal temperature, moisture, light, and nutrient balance, and is represented by adequate growth of both root and shoot tissues such that the growth of one tissue does not dominate at the expense of another tissue during any growth stage. During ideal growth a plant is neither infected by pathogens nor invaded by insects or parasites. An ideally growing plant is generally compact in appearance, with equal amounts of root and shoot mass, good color, and good flower and fruit set. An ideally growing plant will give the maximum yield possible from its genetic potential.
There is a remarkable uniformity of boron requirements and/or boron deficiency symptoms across plant and crop species. The youngest growing tissues are always affected first and in all cases root growth is rapidly impaired. These are the tissues in plants whose regulation and development is also controlled largely by plant hormones. Boron should extend the life and, therefore, the effectiveness of IAA by reducing the breakdown of IAA by IAA-oxidase. Boron has also been shown to increase polyamines, putrescine, spermidine, ascorbic acid, spermine, and the plant hormones, IAA and gibberellic acid. Thus, there is an important interaction/enhancement or synergism between hormones, especially auxin, and boron and other minerals in physiological activity. For example, boron appears to have a direct effect on transport of the plant hormone auxin, possibly by the movement of auxin in and out of cells.
Boron has been shown to be essential for nitrogen fixation by plants, where it enhances the stability of the interconnections between the nodules and the plant roots. Moreover, from an evolutionary standpoint boron-regulated growth may be correlated with the ability of vascular plants to maintain upright growth and to form lignified secondary walls.
Boron deficiency and toxicity inhibit ATPase-dependent hydrogen pumping and ATPase activity in sunflower roots and elicit proton leakage from cells. Thus, membrane activity is strengthened with sufficient and appropriate boron levels through more effective ATPase activity and controlled conductance across the plasma membrane. Borate compounds can inhibit calcium-stimulated ATPase activity as well as store-operated calcium entry channels. Boron enhances phosphorylation and, therefore, signals transduction, including hormone transduction, probably through a mediator whose transduction signals involve a cascade of phosphorylations. It has been reported that boron deficiency reduces oxidative damage to cells and that ascorbate and glutathione levels decrease dramatically with boron deficiency. It has also been suggested that the oxidative damage from boron deficiency is the result of impaired cell wall structure.
Through its effect on proton secretion and on the activity of the plasma membrane NADH oxidase, boron may be directly associated with cell growth. An aploplastic target for the primary action of boron deprivation which signals deeper into the cell via endocytosis-mediated pectin along a putative cell wall plasma membrane cytoskeleton continuum has also been suggested. Boron in animals can act both at the transcriptional and translational level. Further research will likely bear out similar action in plants. Boron is taken up by the plant and accumulates at the growing points where it enters the cell walls. Ninety (90) percent of boron in a plant is in the cell walls in the pectin fraction referred to as the rhamnogalacturonan region where it may be involved in cell to cell adhesion and therefore cell signaling for effective plant growth. Pollen germination is especially sensitive to boron deficiency. It has been suggested that boron has an important role in ionic membrane transport regulation. Boron appears to be most active in the G2/M phase of the cell cycle, i.e., just before and during mitosis when cells divide.
Further derivatives of boron have been reported to have anti-fungal and anti-bacterial activities. Those activities may be strengthened in combination with plant growth regulators, in particular auxin.
Those skilled in the art have longed sought environmentally friendly methods for improving plant growth and crop productivity while also improving the resistance of plants to pathogens and insects. Thus, there has been a long felt, but unfulfilled need, for such methods. The present invention solves those needs.